The present invention relates in general to ultraviolet (UV) lasers. It relates, in particular, to a laser in which radiation at a wavelength between about 200 and 280 nanometers (nm), generated by a first active laser-resonator, is mixed, in an optically-nonlinear crystal located in a second active laser-resonator, with radiation generated by the second laser resonator and having a wavelength between about 900 and 1080 nm, thereby generating radiation having a third wavelength corresponding to the sum-frequency of the first and second wavelengths and having a wavelength between about 175 and 215 nm.
Optical systems are used in optical lithography for patterning or xe2x80x9cwritingxe2x80x9d on photoresist for lithographic masking operations. The resolution of these optical systems is inversely related to the wavelength used for the patterning or writing. In so-called direct-writing systems, where photoresist coated wafers are directly patterned by an optically-steered, focussed, beam of radiation rather than being exposed through a mask. The, the quality of the beam is as important as the wavelength of the beam for obtaining highest possible resolution, and, accordingly smallest possible feature size. Smaller features, of course, lead to higher component packing densities. One particularly useful light source for direct writing operations is an intracavity frequency-doubled argon ion-laser, having an output wavelength of 244 nm. Such a laser is used in a direct writing system manufactured by Etec, Inc., of Hayward, Calif.
Because of a continuing demand for ever smaller and faster semiconductor devices, there is a similar need for a laser system having a shorter wavelength than the 244 nm of existing direct-writing systems. Such a laser system, of course, should have a beam-quality comparable to existing 244 nm laser systems and provide sufficient power such that exposure times are not unacceptably protracted.
In one aspect of the present invention, a laser system comprises first and second active laser-resonators. The first laser-resonator delivers laser-radiation at a first wavelength, and the second laser-resonator is arranged to generate laser-radiation therein at a second wavelength.
The second laser-resonator includes an optically-nonlinear crystal. The first and second laser-resonators and the optically-nonlinear crystal are cooperatively arranged such that first-wavelength radiation delivered by the first laser-resonator is mixed in the optically-nonlinear crystal with the second-wavelength radiation generated in the second laser-resonator, thereby generating radiation having a third-wavelength corresponding to the sum frequency of the first and second wavelengths.
In another aspect of the present invention, radiation having a wavelength between about 200 and 280 nanometers (nm), generated by a first active laser-resonator, is mixed, in an optically-nonlinear crystal located in a second active laser-resonator, with radiation generated by a second laser resonator and having wavelength between about 900 and 1080 nm, thereby generating radiation having a third wavelength corresponding to the sum-frequency of the first and second wavelengths and having a wavelength between about 175 and 215 nm.
In one embodiment of a laser system in accordance with the present invention, 1 Watt (W) of 244 nm radiation is delivered by a prior-art, intracavity frequency-doubled argon-ion laser. Fundamental radiation having a wavelength of about 976 nm is generated in an external-cavity surface-emitting semiconductor laser-resonator (OPS laser-resonator). The 244 nm radiation is mixed in a cesium lithium borate (CLBO) optically-nonlinear crystal with about 500 W of 976 nm radiation circulating in the OPS laser-resonator to provide about 100 milliwatts (mW) of radiation having a wavelength of about 195 nm.
Dispersion characteristics of CLBO allow that the 244 nm radiation can be directed into the CLBO optically-nonlinear crystal without passing through any optical components of the second resonator, thereby avoiding potential absorption losses in those components. Similarly, the 195 nm radiation generated by the sum-frequency mixing, and residual 244 nm radiation, leave the second resonator without passing through any optical components thereof.
In another embodiment of the inventive laser system, the optically-nonlinear crystal in the active, second laser-resonator is commonly located in a passive, travelling-wave ring-resonator ring-resonator arranged to recirculate and build up the 244 nm radiation passing through the CLBO crystal, thereby increasing the output power of 195 nm radiation to about 800 mW.